When considering the range detection that is incident upon an active region of a detector it is important to amplify the electrical output signal, which is a function of the amount of radiation that is incident upon an active region of the detector. Infrared detectors are those detectors, which are sensitive to radiation in the infrared region of the electromagnetic spectrum. There are two types of infrared detectors, thermal detectors including bolometers and photon detectors. The photon detectors function based upon the number of photons that are incident upon and interact with electrons in a transducer region of the detector. The photon detectors, since they function based on direct interactions between electrons and photons, are highly sensitive and have a high response speed compared to the bolometers. However, they have a shortcoming in that the photon detectors operate well only at low temperatures necessitating a need to an incorporate therein an additional cooling system. The bolometers function, on the other hand, based upon a change in the temperature of the transducer region of the detector due to absorption of the radiation. The bolometers provide an output signal, i.e., a change in the resistance of materials (called bolometer elements), that is proportional to the temperature of the transducer region. The bolometer elements have been made from both metals and semiconductors with an increasing effort to combine metal-semiconductor materials. As it is well known in metals, the resistance change is essentially due to variations in the carrier mobility, which typically decreases with temperature. Greater sensitivity can be obtained in high-resistivity metal-semiconductor bolometer elements in which the combination of free-carrier density (i.e., the contribution due to the semiconductor material) along the contribution associated with the metal element undoubtedly increase the performance on the bolometric material.
There is a tremendous effort towards finding alternatives to currently used standard materials in order to enhance the performance of bolometric devices. Materials used in commercial bolometers, like the ones used in infrared imaging systems, require a high temperature coefficient of resistance (TCR), low conductivity and the possibility of performing lithographic patterns on them. The responsivity RV of a bolometer, i.e. the output signal voltage per incident infrared power, is given by
      R    V    =                    I        b            ⁢      R      ⁢                          ⁢      βη              G      ⁢                        1          +                                    ω              2                        ⁢                          τ              2                                          where Ib is the bias current, R is the dc resistance, β is the temperature coefficient of resistance (TCR), η is the absorptivity, G is the thermal conductance between sensitive element and the substrate, ω is the angular modulation frequency of the incident radiation, and τ is the thermal response time which is given by C/G. C is the heat capacity (thermal mass) of the sensitive element. Therefore, a microbolometer requires a temperature sensitive element that displays a high TCR and a structure that has a low thermal conductance and thermal mass. Recently, Zinc oxide has attracted a lot of attention due to its potential to have TCR values higher than Vanadium oxide (VOx) and amorphous Silicon (a-Si), which are the most common materials used in bolometric applications. In addition to its potentially high TCR values, the optical properties of ZnO are also important due to its wide bandgap of 3.37 eV with a large excitation binding energy of 60 meV.
There remains a need for improved bolometric materials and methods of making thereof.